Method of fabricating an ink jet print head

ABSTRACT

An ink jet print head comprises a piezoelectric plate (2) formed of a piezoelectric material, a base plate (1) formed of a nonconductive, nonelectrostrictive material having rigidity lower than that of the piezoelectric material and joined to the piezoelectric plate (2), electrodes (8) formed by depositing a metal by electroless plating over the entire bottom surfaces of a plurality of parallel grooves (3) formed through the piezoelectric plate (2) into the base plate (1) and the entire side surfaces of side walls (4) each consisting of an upper side wall (4a) formed in the piezoelectric plate (2) and lower side wall (4b) formed in the base plate (2) between the grooves (3), a top plate (10) joined to the upper surface of the piezoelectric plate (2) so as to close the upper open ends of the grooves (3) to form pressure chambers (14), and a nozzle plate (12) provided with ink jets (11) and joined to one end of the assembly of the base plate (1), the piezoelectric plate (2) and the top plate (10) so that the ink jets (11) correspond respectively to the pressure chambers (14).

This is a division, of application Ser. No. 07/853,267, filed on Mar.18, 1992 now U.S. Pat. No. 5,311,218.

FIELD OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to an on-demand ink jet print head and amethod of manufacturing the same.

FIG. 10 shows an ink jet print head of an invention disclosed inJapanese Patent Laid-open (Kokai) No. Hei 2-150355. Referring to FIG.10, a bottom sheet 30 having a polarity indicated by the arrows isprovided with a plurality of parallel grooves 31 defined by side walls32 and a bottom wall 33. A top sheet 35 is attached adhesively by anadhesive layer 36 to the upper ends 34 of the side walls 32 to close theopen upper end of the grooves 31. Upper portions of the side surfaces ofthe side walls 32, namely, the side surfaces of each groove 31, of alength corresponding to substantially half the depth of the groove 31are metallized by evaporation to form electrodes 37.

The bottom sheet 30 is held on a jig in a vacuum evaporation apparatusand parallel atomic beams of a metal are projected on one side surfaceof each side wall 32 of the bottom sheet 30 at an angle δ to the sameside surface of each side wall 32 as shown in FIG. 11 to deposit a metalfilm, i.e., the electrode 37, on the side surface of each side wall 32.Then, the bottom sheet 30 is turned through an angle of 180° in ahorizontal plane, as viewed in FIG. 11, and the bottom sheet 30 issubjected to the same vacuum evaporation process to deposit a metalfilm, i.e., the electrode 37, on the other side surface of each sidewall 32. Thus, the electrodes 37 are formed by evaporation on therespective upper halves of the opposite side surfaces of each side wall32. Metal films deposited on the upper ends 34 of the side walls 32 areremoved in the next process.

The grooves 31 are closed by the top sheet 35 to form pressure chambers.Then, an ink inlet opening to be connected to an ink supply unit isformed in one end of each pressure chamber, and an ink jet through whichink is jetted is formed in the other end of the pressure chamber tocomplete an ink jet print head.

When voltages of opposite polarities are applied to the electrodes 37 ofthe two adjacent side walls 32, shearing strains as indicated by dottedlines in FIG. 10 result from a potential of a direction perpendicular tothe direction of polarity of the bottom sheet 30 indicated by the arrowsacting on the side walls 32. Consequently, the volume of the pressurechamber (the groove 31) between the sheared side walls 32 is reducedinstantaneously and thereby the internal pressure of the pressurechamber is increased sharply to jet the ink through the ink jet.

FIGS. 12(a) and 12(b) show an ink jet print head of an inventiondisclosed in Japanese Patent Laid-open (Kokai) No. Sho 63-247051.Referring to FIG. 12(a), a bottom wall 38, a hard side wall 39, a topwall 40 and an actuator 41 are combined so as to form a passage 42. Theactuator 41 is formed of a piezoelectric ceramic and is polarized in adirection along a Z-axis. A strip seal 43 is attached to the upper endof the actuator 41 so as to be held between the actuator 41 and the topwall 40. The lower end of the actuator 41 is joined to the bottom wall38. Electrodes 44 and 45 are formed on the opposite side surfaces of theactuator 41. A nozzle 46 is provided at the front end of the passage 42.When ink is supplied from an ink supply unit into the passage 42 and anelectric field is applied to the electrodes 44 and 45, the actuator 41is strained as shown in FIG. 12(b) to compress the passage 42 and,consequently, the ink 46 is jetted through the nozzle 46.

The ink jet print head disclosed in Japanese Patent Laid-open (Kokai)No. Hei 2-150355 has the following four disadvantages.

First, the side walls 32 cannot sufficiently be strained (deformed). Theside wall 32 is strained by an electric field of a directionperpendicular to the direction of polarization of the bottom sheet 30created by applying a voltage across the opposite electrodes 37 formedon the opposite side surfaces of the groove 31. Then, the strain of theupper half portion of the side wall 32 provided with the electrodes 37is sustained by the lower half portion of the same not provided with anyelectrode 37. Accordingly, the lower half portion of the side wall 32acts as a resistance against the straining of the upper half portion ofthe same side wall 32. Since the side wall 32 is a solid body formed ofsingle material (piezoelectric material) and having a high rigidity, itis impossible to strain the side wall 32 greatly and hence the variationin the volume of the pressure chamber is relatively small.

Secondly, the ink jet print head requires a costly process for formingthe electrodes 37. Since the electrodes 37 must be formed only in theupper half portions of the side surfaces of the side walls 32, a specialvacuum evaporation apparatus having a complicated construction must beused for forming the electrodes 37. Furthermore, the process of formingthe electrodes 37 must be carried out in a plurality of steps ofprojecting the parallel atomic beams of a metal on one side surface ofeach side wall 32 at the predetermined angle δ to the side surface toform the electrode 37 on one side surface of each side wall 32, turningthe bottom sheet 30 through an angle of 180° in a horizontal plane, andprojecting the parallel atomic beams of a metal again on the other sidesurface of each side wall 32 at the predetermined angle δ to the sidesurface to form the electrode 37 on the other side surface of each sidewall 32.

Thirdly, it is impossible to apply an electric field uniformly to thebottom sheet 30 formed of a piezoelectric material. A piezoelectric workfor forming the bottom sheet 30, in general, is a sintered workconsisting of crystal grains. Therefore, crystal grains appear in theside surfaces of the grooves 31 finished by grinding to formirregularities in the side surfaces of the groove 31. On the other hand,in forming the electrodes 37, no metal is deposited on portions of theside surfaces of the grooves 31 not facing directly to the atomic beamprojecting source of the vacuum evaporation apparatus. Accordingly, themetal is deposited only on projections in the ground side surfaces ofthe grooves 31 and pinholes are formed at positions corresponding topits between the projections, which makes it impossible to apply anelectric field uniformly to the bottom sheet 30.

Fourthly, the ground side surfaces of the grooves 31 are subject to thecorrosive action of the ink and hence the ground side surfaces of thegrooves 31 must be coated with a protective film, which, however, isdifficult. The ground side surfaces of the grooves 31 of the sinteredbottom sheet 30 consisting of crystal grains is subject to the corrosiveaction of the ink. However, it is possible to coat the side surfaces ofthe groove 31 partially with the electrodes 37 having many pinholes, andhence the electrodes 37 are unable to serve as satisfactory protectivefilms.

The ink jet print head disclosed in Japanese Patent Laid-open (Kokai)No. Sho 63-247051 has disadvantages likewise.

Firstly, many strip seals 43 having a shape corresponding to thesectional shape of the actuators 41 must be attached to the upper endsof the actuators 41, which requires much time and labor.

Secondly, although the inner surfaces of the bottom wall 38, the hardside wall 39 and the actuator 41 are exposed to the ink, no protectivemeasure is taken to protect the inner surfaces from the corrosive actionof the ink. The top wall 40 can be formed of a corrosion-resistantmaterial chosen among relatively many possible materials and the surfaceof the plate-shaped top wall 40 can relatively easily be coated with aprotective film. However, the bottom wall 38, the hard side wall 39 andthe actuator 41 are formed by forming the passage 42 in a solidpiezoelectric ceramic block, and the electrode 45 must be formed on theinner surface of the actuator 41. Only a possible process of forming theelectrode 45 on the inner surface of the actuator 41 may be, in view ofthe size of the passage 42, is a vacuum evaporation process or asputtering process. Accordingly, pinholes are formed inevitably in theelectrode 45. The bottom wall 38 and the hard side wall 39 are exposedto the corrosive action of the ink. Such problems may be solved if theinner surfaces are coated with a protective film. However, it isimpossible to coat the irregular inner, surfaces of the bottom wall 38,the hard side wall 39 and the actuator 41 entirely with a protectivefilm by an ordinary vacuum evaporating process or a sputtering process,because the metal is deposited only on surfaces directly facing thesource metal.

OBJECT AND SUMMARY OF THE INVENTION

Accordingly, it is a first object of the present invention to provide anink jet print head provided with pressure chambers having a large volumecontraction ratio.

A second object of the present invention is to provide an ink jet printhead facilitating the formation of electrodes therein.

A third object of the present invention is to provide an ink jet printhead provided with electrodes having few pinholes.

A fourth object of the present invention is to provide an ink jet printhead having pressure chambers defined by surfaces effectively coatedwith a protective film.

An ink jet print head in a first aspect of the present inventioncomprises: a piezoelectric plate formed of a piezoelectric material,polarized in the direction of its thickness and provided with aplurality of slots separated from each other by upper side walls; a baseplate formed of a nonconductive, nonelectrostrictive material havingrigidity lower than that of the piezoelectric material forming thepiezoelectric plate, provided with grooves separated from each other bylower side walls and joined to the piezoelectric plate so that thegrooves are aligned respectively with the slots of the piezoelectricplate and the lower side walls are connected respectively to the upperside walls to form side walls to form pressure chambers; a plurality ofelectrodes each formed over the entire bottom surfaces and the sidesurfaces of the side walls; a top plate joined to the upper surface ofthe piezoelectric plate so as to seal the pressure chambers; and anozzle plate provided with a plurality of ink jets and joined to one endof the assembly of the base plate, the piezoelectric plate and the topplate so that the ink jets correspond respectively to the pressurechambers. The volume of the pressure chamber is reduced to increase theinternal pressure of the pressure chamber to jet the ink through the inkjet by applying a voltage to the electrodes so that the side walls ofthe pressure chamber are deformed. Since the upper side wall, i.e., oneportion of the side wall on the side of the top plate, is formed of thepiezoelectric material having a high rigidity and the lower side wall,i.e., the other portion of the side wall on the side of the base plate,is formed of a material having a rigidity lower than that of thepiezoelectric material, the resistance of the lower side wall againstthe deformation of the upper side wall is relatively low, so that theside wall is able to be deformed greatly to jet the ink effectively.Since the respective lower side walls of the side walls defining eachpressure chamber are formed of a nonelectrostrictive material, only theupper side walls of the side walls formed of the piezoelectric materialcan be subjected to the action of an electric field even if theelectrode is formed over the entire surfaces of the side walls definingthe pressure chamber. Accordingly, a complicated process of forming anelectrode only in a portion of the surfaces defining the pressurechamber can be eliminated.

An ink jet print head in a second aspect of the present inventioncomprises: a piezoelectric plate formed of a piezoelectric material,polarized in the direction of its thickness and provided with aplurality of slots separated from each other by upper side walls; a baseplate formed of a nonconductive, nonelectrostrictive material havingrigidity lower than that of the piezoelectric material forming thepiezoelectric plate, provided with parallel grooves separated from eachother by lower side walls, and joined to the piezoelectric plate so thatthe grooves are aligned respectively with the slots of the piezoelectricplate and the lower side walls are connected respectively to the upperside walls to form side walls and pressure chambers; electrodes formedover the entire bottom surfaces of the grooves and the entire sidesurfaces of the side walls by electroless plating; a top plate joined tothe upper surface of the piezoelectric plate so as to seal the pressurechambers; and a nozzle plate provided with a plurality of ink jets andjoined to one end of the assembly of the base plate, the piezoelectricplate and the top plate so that the ink jets correspond respectively tothe pressure chambers. The slots of the piezoelectric plate and thegrooves of the base plate are formed by grinding after joining togetherthe piezoelectric plate and the base plate so that the internalstructure of the nonelectrostrictive material forming the base plate isexposed in the ground surfaces of the grooves. Accordingly, both thesurfaces of the slots of the piezoelectric plate and the base plates cansimultaneously be subjected to pretreatment and electroless plating informing the electrodes and hence the electrodes can be formed byelectroless plating at a reduced cost. Since the electrodes formed byelectroless plating have few pinholes and a uniform thickness even ifthe ground surfaces of the grooves are irregular, an electric field canuniformly be applied to the piezoelectric plate. Since the electrodeshaving few pinholes can be formed over the entire surfaces defining thepressure chambers, the surfaces defining the pressure chambers can beprotected from the corrosive action of the ink and any additionalprotective film can be omitted to curtail the cost of the ink jet printhead.

An ink jet print head in a third aspect of the present invention issimilar in construction to the ink jet print head in the second aspectof the present invention. This ink jet print head employs a plasticsubstrate formed of plastic containing a catalyst for electrolessplating. The catalyst promotes the deposition of the metal to improvethe adhesion of the electrodes formed by electroless plating to the baseplate and the piezoelectric plate.

An ink jet print head in a fourth aspect of the present invention issimilar in construction to the ink jet print head in the second aspectof the present invention. A plurality of grooves are formed through thepiezoelectric plate into the base plate by grinding, a catalyst forelectroless plating is applied to the surfaces of the grooves, a maskcovering portions of the surface of the piezoelectric plate other thanthose in which the electrodes and a wiring pattern of conductive filmare to be formed is formed by a photolithographic process, the assemblyof the piezoelectric plate and the base plate is immersed in anelectroless plating bath to form the electrodes and the wiring patternof a conductive film simultaneously. Accordingly, the steps of formingthe electrodes and the wiring pattern are reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of an ink jet print head in afirst embodiment according to the present invention;

FIG. 2 is a timing chart of assistance in explaining a manner ofapplying a voltage to the electrode of the ink jet print head of FIG. 1;

FIGS. 3(a), 3(b) and 3(c) are perspective views of assistance inexplaining steps of fabricating the ink jet print head of FIG. 1;

FIGS. 4(a) and 4(b) are perspective views of assistance in explainingsteps of fabricating the ink jet print head of FIG. 1;

FIGS. 5(a), 5(b) and 5(c) are perspective views of assistance inexplaining steps of fabricating the ink jet print head of FIG. 1;

FIG. 6 is a fragmentary perspective view of assistance in explaining thedimensions of side walls of the ink jet print head of FIG. 1;

FIG. 7 is a graph showing the variation of strain in a piezoelectricplate with the thickness of the piezoelectric plate for the elasticconstant of the piezoelectric plate;

FIG. 8 is a graph showing the variation of shearing force with thethickness of the piezoelectric plate for the elastic constant of thepiezoelectric plate;

FIG. 9 is a graph showing the variation of shearing energy with thethickness of the piezoelectric plate for the elastic constant of thepiezoelectric plate;

FIG. 10 is a longitudinal sectional view of a conventional ink jet printhead;

FIG. 11 is a side view of assistance in explaining a manner of formingelectrodes; and

FIGS. 12(a) and 12(b) are longitudinal sectional views of anotherconventional ink jet print head.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An ink jet print head in a first embodiment according to the presentinvention will be described with reference to FIGS. 1 to 9. First,referring to FIG. 3(a), a piezoelectric plate 2 formed of apiezoelectric material and polarized in the direction of its thicknessis joined adhesively with an adhesive to the upper surface of a baseplate 1 formed of a nonconductive, nonelectrostrictive material having arigidity lower than that of the piezoelectric material forming thepiezoelectric plate 2. The nonconductive, nonelectrostrictive materialforming the base plate 1 employed in this embodiment is a liquid crystalpolymer (ZAITER®, Nippon Sekiyu Kagaku K.K.). The adhesive is anonconductive industrial adhesive. Bubbles contained in the adhesivereduces the adhesive strength of the adhesive and hence, if necessary,the adhesive is deaerated. The thickness of the film of the adhesive is,desirably, on the order of 1 μm. The characteristics of thepiezoelectric plate 2 is deteriorated if the same is heated above apredetermined temperature because the piezoelectric plate 2 ispolarized. Therefore, in adhesively joining together the base plate 1and the piezoelectric plate 2, an adhesive capable of hardening at ahardening temperature that will not deteriorate the characteristics ofthe piezoelectric plate 2 is desired. The adhesive employed in thisembodiment is SCOTCH WELD 1838B/A® (Sumitomo 3M K.K.).

Referring to FIG. 3(b), a plurality of parallel grooves 3 are formed atpredetermined intervals through the piezoelectric plate 2 and in thebase plate 1 by grinding. Before forming the grooves 3 by grinding, thebottom surface of the base plate 1 is ground with reference to thesurface of the piezoelectric plate 2 to finish the assembly of the baseplate 1 and the piezoelectric plate 2 in a predetermined thickness, thebase plate i is fixed to the bed of a grinding machine, and the feed ofthe grinding machine is determined with reference to the surface of thebed to form the grooves 3 in a predetermined depth. Naturally, the depthof the grooves 3 may be determined with reference to the surface of thepiezoelectric plate 2 to omit the process of grinding the bottom surfaceof the base plate 1. The grooves 3 are separated from each other by sidewalls 4. Each side wall 4 consists of an upper side wall 4a formed ofthe piezoelectric material of the piezoelectric plate 2, and a lowerside wall 4b having a rigidity lower than that of the upper side wall4a. The grooves 3 are 80 μm in width and 160 μm in depth, and the pitchof the grooves 3 is 169 μm. Generally, a diamond wheel employed in adicing saw for dicing wafers to provide IC chips is used for forming thegrooves 3. In this embodiment, a 2 in. diameter diamond wheel NBCZ1080®or NBCZ1090® (K.K. Disuko) is used. The diamond wheel is rotated at30,000 rpm in forming the grooves 3. Since the base plate 1 is formed ofthe liquid crystal polymer, the grooves 3 can be formed without formingany burr.

The work consisting of the base plate 1 and the piezoelectric plate 2 issubjected to pretreatment before forming electrodes by electrolessplating. The surfaces of the work is etched for thirty minutes by apotassium hydroxide solution of 30% in concentration heated at 50° C. tofinish the surfaces of the grooves 3 in a roughness capable of securinga sufficiently high adhesion of the plated film to the surfaces of thegrooves 3. Then, the work is subjected to a cleaning and conditioningprocess using a cationic surface active agent for degreasing and forimproving the catalyst adsorbing property of the surfaces of the grooves3. Then, the work is subjected to a pretreatment process for applying acatalyst to the surfaces of the work. In this pretreatment process, thework is immersed in a catalyst solution containing neutral salt, such asNaCl, Pd and Sn, the work is treated by an acid accelerator, so thatonly Pd as a catalyst remains over the surfaces of the work, and thenthe work is dried. It is desirable to employ an ultrasonic device tomake the solution permeate the surfaces of the grooves 3 perfectly.

Then, a mask is formed over the surface of the piezoelectric plate 2.The mask covers portions of the surface of the piezoelectric plate 2other than those in which electrodes and a wiring pattern of aconductive film are to be formed. A dry film 5 is formed over thesurface of the piezoelectric plate 2 as shown in FIG. 3(c), a mask 6 isplaced on the dry film 5 as shown in FIG. 4(a) and the dry film 5 isexposed to light and the exposed dry film 5 is subjected to developing.Resist films 7 are formed over the surface of the piezoelectric plate 2excluding portions in which electrodes and a wiring pattern of aconductive film are to be formed. The surfaces of the portions in whichelectrodes and a wiring pattern of a conductive film are to be formedare coated with Pd, i.e., a catalyst.

Then, the work is immersed in a plating bath for electroless plating.The portions of the surface of the work other than those in whichelectrodes and a wiring pattern are to be formed are isolated from theplating bath by the resist films 7. Suitable metals to be deposited byelectroless plating are gold and nickel. The plating bath containsmetallic salt and a reducing agent as principal components, andadditives, such as a pH regulator, a buffer, a complexing agent, anaccelerator, a stabilizer, a modifier and the like. In this embodiment,a low-temperature Ni-P plating bath is used. A layer of metal is formedby electroless plating in a thickness in the range of 2 to 3 μm. Sinceelectroless plating, differing from electroplating, is a chemicalprocess, the mode of deposition of the metal can simply be controlled byregulating the pH and the concentration of the components of the platingbath. When the work is immersed in the plating bath, Pd (catalyst)spread over the surface of the portions not coated with the resist films7 acts as a catalyst and the metal is deposited in those portions of thesurface of the work. After Pd has been coated with a film of thedeposited metal, the autocatalysis of the deposited metal promoteselectroless plating. When the metal is deposited in a film of a desiredthickness, the electroless plating process is terminated. Thus,electrodes 8 are formed over the entire side surfaces of the side walls4 defining the grooves 3 and not coated with the resist film 7, and awiring pattern 9 continuous with the electrodes 8 is formed in theportions of the surface of the piezoelectric plate 2 not coated with theresist film 7 as shown in FIG. 5(a). Since the plating bath permeatesthe minute structure of the surface of the base plate 1 and thepiezoelectric plate 2 and few pinholes are formed in the films of thedeposited metal, the side surfaces of the side walls 4 and the film ofthe adhesive, which is not sufficiently resistant to water, formedbetween the base plate 1 and the piezoelectric plate 2 defining thegrooves 3 are protected from the ink. Accordingly, any additionalprotective film is unnecessary. The electrodes 8 and the wiring pattern9 are formed in a uniform thickness.

Then, as shown in FIG. 5(b), the resist films 7 are removed from thesurface of the piezoelectric plate 2.

Then, as shown in FIG. 5(c), a top plate 10 is attached adhesively tothe upper surface of the piezoelectric plate 2. Since the resist films 7of about 20 μm in thickness, which is thicker than the metal film formedby electroless plating, have been removed, the top plate 10 cansatisfactorily be attached to the upper surface of the piezoelectricplate 2. A nozzle plate 12 provided with a plurality of ink jets 11 isattached to one end of the assembly of the base plate 1, thepiezoelectric plate 2 and the top plate 10 so that the ink jets 11correspond respectively to the grooves 3 to complete the ink jet printhead. An ink supply pipe 13 is joined to the top plate 10 to connect thegrooves 3 to an ink supply unit, not shown. As shown in FIG. 1, therespective upper ends of the grooves 3 are closed by the top plate 10 toform pressure chambers 14.

Operation of the ink jet print head thus constructed in jetting the inkfrom the middle pressure chamber 14, as viewed in FIG. 1, will bedescribed hereinafter. The pressure chambers 14 are filled up with theink supplied through the ink supply pipe 13 from the ink supply unit. Avoltage A is applied through the wiring pattern 9 across the electrode 8of the middle pressure chamber 14 and the electrode of the left pressurechamber 14 on the left-hand side of the middle pressure chamber 14, anda voltage B of a polarity reverse to that of the voltage A is appliedthrough the wiring pattern 9 across the electrode 8 of the middlepressure chamber 14 and the electrode 8 of the right pressure chamber 14on the right-hand side of the middle pressure chamber 14 to apply anelectric field of a direction perpendicular to the direction ofpolarization indicated by the arrows to the upper side walls 4a.Consequently, the side wall 4 on the left-hand side of the middlepressure chamber 14 is strained to the left and the side wall 4 on theright-hand side of the middle pressure chamber 14 is strained to theright to increase the volume of the middle pressure chamber 14 and toreduce the respective volumes of the pressure chambers 14 on theopposite sides of the middle pressure chamber 14.

Since the voltages A and B are increased gradually in a fixed timeperiod a as shown in FIG. 2, the ink is not jetted through the ink jets11 of the right and left pressure chambers 14, though the respectivevolumes of the right and left pressure chambers 14 are reduced. Thelevel of the ink in the middle pressure chamber 14 is lowered slightlywhen the volume of the middle pressure chamber 14 is increased, andthen, the ink is sucked through the ink supply pipe 13 into the middlepressure chamber 14. The polarities of the voltages A and B are reversedinstantaneously at time b (FIG. 2) to strain instantaneously the sidewall 4 on the left-hand side of the middle pressure chamber 14 to theright and the side wall 4 on the right-hand side of the middle pressurechamber 14 to the left. Consequently, the volume of the middle pressurechamber 14 is reduced sharply to jet the ink through the ink jet 14 ofthe middle pressure chamber 14. The voltages A and B of the reversepolarities are maintained for a predetermined time period c (FIG. 2).While the ink is thus jetted through the ink jet 11, the droplet of theink jetted through the ink jet 11 is continuous with the ink jet 11. Attime d, the voltages A and B are removed instantaneously from theelectrodes 8 to allow the strained side walls 4 to restore theiroriginal shapes rapidly. Consequently, the internal pressure of themiddle pressure chamber 14 drops sharply and thereby a rear portion ofthe ink droplet flying in the vicinity of the ink jet 11 is separatedfrom the ink droplet on the axis of the ink jet 11 and is sucked intothe middle pressure chamber 14. Thus, the ink droplet flies in a fixeddirection and is not separated into a plurality of smaller ink dropletswhich form satellite dots. Although the internal pressures of the rightand left pressure chambers 14 increase at the moment when the voltages Aand B are removed from the electrodes 8, the internal pressures do notincrease to a pressure level high enough to jet the ink through the inkjets 11.

Thus, the upper side walls 4a of the side walls 4 are portions of thepiezoelectric plate 2 formed of a piezoelectric material having highrigidity and the lower side walls 4b of the side walls 4 are portions ofthe base plate 1 formed of a material having a rigidity lower than thatof the piezoelectric material forming the piezoelectric plate 2.Therefore, the upper side walls 4a can be strained greatly without beingobstructed significantly by the lower Side walls 4b to enhance the inkjetting characteristic of the ink jet print head.

Incidentally, suppose that each side wall 4 has a height h (the depth ofthe groove 3) of 160 μm, a width B of 80 μm and a length L of 10 mm asshown in FIG. 6 and

    d.sub.15 =564×10.sup.-12 m/V

    S.sub.44 =37.4×10.sup.-12 m.sup.2 /N

where d₁₅ is the piezoelectric constant of the piezoelectric plate 2 andS₄₄ is the elastic constant of the piezoelectric plate 2.

The variation of the strain of the side wall 4 (FIG. 7), the variationof shearing force acting on the side wall 4 (FIG. 8) and the variationof strain energy stored in the side wall 4 with the thickness y of thepiezoelectric plate 2 (FIG. 9) for the elastic constant (the reciprocalof rigidity) of the base plate 1 will be examined, In FIGS. 7, 8 and 9,curves for S_(p) =37.4×10⁻¹² m² /N represent the characteristics of theside wall of the conventional ink jet print head, in which the side wallis formed entirely of the material forming the piezoelectric plate. Asis obvious from FIG. 7, the strain of the side wall 4 is larger, namely,the efficiency of straining the side wall 4 is higher, for the largerelastic constant S_(p) of the base plate 1. Thus, the elastic constantS_(p) of the base plate 1, the height h of the side wall 4 (the depth ofthe groove 3) and the thickness y of the piezoelectric plate 2 aredetermined selectively to obtain an ink jet print head having optimumstrain, shearing and energy characteristics.

Referring to FIG. 9, every energy-thickness curve for elastic constantS_(p) of the base plate 1 has a maximum. In FIG. 9, a curve indicated atA passes the maxima of the curves. The thickness y of the piezoelectricplate 2 corresponding to the maximum is expressed as a function of theheight h of the side wall 4 (the depth of the groove 3), the elasticconstant S₄₄ of the piezoelectric plate 2 and the elastic constant S_(p)(the reciprocal of the rigidity) of the base plate 1. ##EQU1##

The piezoelectric plate 2 is designed in a thickness approximately equalto the thickness y calculated by using this expression to obtain an inkjet print head provided with side walls 4 capable of being deformedgreatly, and having an enhanced ink jet characteristic.

Since the portions of the side walls 4, namely, the lower side walls 4b,are formed of the nonelectrostrictive material, an electric field actsonly on the upper side walls 4a formed in the piezoelectric plate 2 evenif the electrodes 8 are formed over the bottom surfaces of the grooves 3and the side surfaces of the lower side walls 4b, which are formed inthe base plate 1. Therefore, the electrodes 8 can be formed by anelectroless plating process which is less costly than an electroplatingprocess. Electroless plating is capable of forming the electrodes 8having few pinholes in a uniform thickness over the irregular groundsurfaces of the grooves 3. Therefore, an electric field can be applieduniformly to the piezoelectric plate 2. The surfaces of the pressurechambers 14 formed in the base plate 1 and the piezoelectric plate 2 areprotected from the corrosive action of the ink by the electrodes 8having few pinholes and entirely coating the surfaces of the pressurechambers 14. Therefore, additional protective films may be omitted toreduce the cost of the ink jet print head.

In a modification, the base plate 1 was formed of a PPS resin instead ofthe liquid crystal polymer. The side walls 4 of the ink jet print headin this modification could be strained greatly. When the base plate 1was formed of the PPS resin, the work consisting of the base plate 1 andthe piezoelectric plate 2 was etched by an etching solution containingtin fluoride and additives at 25° C. for about thirty minutes forpretreatment before forming the electrodes 8 by electroless plating. ANi-B plating bath was used for electroless plating.

An ink jet print head in a second embodiment according to the presentinvention employs a base plate 1 formed of engineering plasticcontaining metal powder which serves as a catalyst for electrolessplating. When grooves 3 are formed in the base plate 1 by grinding, thecatalyst is exposed in the surfaces of the grooves 3 and the metal isdeposited on the catalyst, which enhances the adhesion of the metaldeposited on the surfaces of the grooves 3. The metal powder may be Pdpowder, Rh powder, Ag powder or Au powder. Satisfactory electrodes 8 canbe formed by electroless plating when the PPS resin forming the baseplate 1 contains 2 to 5% by weight of Pd powder of 1 μm or less grainsize.

Possible materials for forming the base plate 1 are not limited to theforegoing materials; the base plate 1 may be formed of any suitablematerial, provided that the material is nonconductive andnonelectrostrictive, the rigidity of the material is lower than that ofthe material forming the piezoelectric plate 2, the base plate 1 formedof the material can be attached adhesively to the piezoelectric plate 2,the surfaces of the grooves 3 of the base plate 1 formed of the materialcan be finished by grinding with a diamond wheel in smooth surfaces, andthe metal for forming the electrodes 8 can be deposited in high adhesionby electroless plating over the surfaces of the grooves 3 when the baseplate 1 and the piezoelectric plate 2 are subjected simultaneously toelectroless plating. The electrodes 8 may be formed of inexpensive Ni,however, if the Ni electrodes 8 are subject to the corrosive action ofthe ink, electrodes 8 may be formed of gold. To suppress an increase incost of the ink jet print head, the electrodes 8 may be formed bydepositing a Ni film and coating the Ni film with a thin film of gold.

Thus, the ink jet print head in the first embodiment according to thepresent invention comprises: the piezoelectric plate formed of apiezoelectric material, polarized in the direction of its thickness andprovided with the plurality of parallel slots separated from each otherby the upper side walls; the base plate formed of a nonconductive,nonelectrostrictive material having rigidity lower than that of thepiezoelectric material forming the piezoelectric plate, provided withparallel grooves separated from each other by lower side walls andjoined to the piezoelectric plate so that the grooves are alignedrespectively with the slots of the piezoelectric plate and the lowerside walls are connected respectively to the upper side walls so as toform side walls to form pressure chambers; the plurality of electrodesformed over the entire bottom surfaces of the grooves and the entireside surfaces of the side walls; the top plate joined to the uppersurface of the piezoelectric plate so as to seal the pressure chambers;and the nozzle plate provided with the plurality of ink jets andattached to one end of the assembly of the base plate, the piezoelectricplate and the top plate so that the ink jets correspond respectively tothe pressure chambers. The side walls are deformed by applying voltagesacross the electrodes so as be reduce the volume of the pressure chamberto increase the internal pressure of the pressure chamber, so that theink is jetted through the ink jet. Since a portion, i.e., the lower sidewall, of each of the side walls on the opposite sides of the pressurechamber is formed of a nonelectrostrictive material, an electric fieldcan be applied only to the other portion, i.e., the upper side wall, ofthe side wall even if the electrodes are formed over the entire sidesurfaces of the side walls each consisting of the upper side wall andthe lower side wall. Accordingly, the complicated process for formingthe electrode only on a portion of the surface of each side wall can beomitted.

The ink jet print head in the second embodiment according to the presentinvention comprises: the piezoelectric plate formed of a piezoelectricmaterial, polarized in the direction of its thickness and provided withthe plurality of slots separated from each other by the upper sidewalls; the base plate formed of a nonconductive, nonelectrostrictivematerial having rigidity lower than that of the piezoelectric materialforming the piezoelectric plate, provided with the grooves separatedfrom each other by lower side walls and joined to the piezoelectricplate so that the grooves are aligned respectively with the slots of thepiezoelectric plate and the lower side walls are connected respectivelyto the upper side walls to form the pressure chambers; the electrodesformed over the entire bottom surfaces of the grooves and the entireside surfaces of the side walls by electroless plating; the top platejoined to the upper surface of the piezoelectric plate to seal thepressure chambers; and a nozzle plate provided with the ink jets andjoined to one end of the assembly of the base plate, the piezoelectricplate and the top plate so that the ink jets correspond respectively tothe pressure chambers. Accordingly, the side surfaces of the lower sidewalls formed in the nonelectrostrictive base plate and the side surfacesof the upper side walls formed in the piezoelectric plate cansimultaneously be subjected to the pretreatment and the subsequentelectroless plating. Thus, the electrodes can be formed at a reducedcost by electroless plating capable of forming the electrodes having fewpinholes uniformly even if the ground side surfaces of the upper sidewalls are irregular. Therefore, an electric field can uniformly beapplied to the piezoelectric plate. Furthermore, since the bottomsurfaces of the grooves and the side surfaces of the side walls definingthe pressure chambers can be coated entirely with the electrodes havingfew pinholes, the surfaces defining the pressure chambers can beprotected from the corrosive action of the ink and any additionalprotective film need not be formed, which reduces the cost of the inkjet print head.

The ink jet print head in the third embodiment according to the presentinvention having the construction similar to that of the ink jet printhead in the second embodiment employs the base plate formed of a plasticcontaining a catalyst for electroless plating. Accordingly, theelectrodes having enhanced adhesion can be formed by electrolessplating.

In forming the electrodes over the ground surfaces of the side wallseach consisting of the upper side wall formed in the piezoelectric plateand the lower side wall formed in the base plate of the ink jet printhead in the fourth embodiment according to the present invention havingthe construction similar to those of the ink jet print heads in thesecond and third embodiments, a catalyst for electroless plating isapplied to the same ground surfaces, a mask is formed by aphotolithographic process so as to coat portions of the surface of thepiezoelectric plate, other than those in which the electrodes and thewiring pattern are to be formed, and the work consisting of the baseplate and the piezoelectric plate is immersed in an electroless platingbath to form the electrodes and the wiring pattern simultaneously.Accordingly, steps of fabricating the ink jet print head can be reduced.

What is claimed is:
 1. A method of fabricating an ink jet print head,comprising steps of:joining together a piezoelectric plate formed of apiezoelectric material and polarized in a direction of its thickness anda base plate formed of a nonconductive, nonelectrostrictive materialhaving a rigidity lower than a rigidity of the piezoelectric materialforming the piezoelectric plate; forming a plurality of parallel groovesthrough the piezoelectric plate into the base plate by grinding so thatan internal structure of the base plate is exposed in the groundsurfaces of side walls between the grooves; forming metal layers overentire bottom surfaces of the grooves and entire side surfaces of theside walls by electroless plating to form electrodes; joining a topplate to an upper surface of the piezoelectric plate so as to close thegrooves to form pressure chambers; and attaching a nozzle plate providedwith a plurality of ink jets corresponding respectively to the pressurechambers to one end of the assembly of the base plate, the piezoelectricplate and the top plate.
 2. A method of fabricating an ink jet printhead according to claim 1, wherein the step of forming metal layers overentire bottom surfaces of the grooves and entire side surfaces of theside walls further comprises the steps of applying a catalyst to theentire bottom surfaces of the grooves and the entire side surfaces ofthe side walls between the grooves, forming a mask by aphotolithographic process so as to coat portions of the surface of thepiezoelectric plate other than portions of the same in which theelectrodes and a wiring pattern are to be formed, and immersing theassembly of the piezoelectric plate and the base plate in an electrolessplating bath after forming the mask to form the electrodes and a wiringpattern simultaneously.
 3. A method of fabricating an ink jet print headaccording to claim 2, wherein the material forming the base plate isplastic containing a catalyst for electroless plating.
 4. A method offabricating an ink jet print head according to claim 3, wherein the stepof forming metal layers over entire bottom surfaces of the grooves andentire side surfaces of the side walls further comprises the steps ofapplying a catalyst to the entire bottom surfaces of the grooves and theentire side surfaces of the side walls between the grooves, forming amask by a photolithographic process so as to coat portions of thesurface of the piezoelectric plate other than portions of the same inwhich the electrodes and a wiring pattern are to be formed, andimmersing the assembly of the piezoelectric plate and the base plate inan electroless plating bath after forming the mask to form theelectrodes and a wiring pattern simultaneously.